This invention relates to shock absorbers for vehicle use of the type including a combination of a hydraulic cylinder and a piston slidably fitted therein and having freely adjustable damping characteristics for expansion and compression stroke movements of the assembly.
In general, it is desirable that a shock absorber usable in the suspension system of a vehicle has damping characteristics that can be adjusted as desired according to various service conditions such as the driver's taste in ride, the load condition of the vehicle and the road surface conditions. This requirement is particularly strong for small-sized vehicles of limited springing weight such as motorcycles but in practice it has previously been difficult to make adjustable the damping characteristics of a shock absorber as this tends to cause substantial increase in structural complexity and manufacturing cost.
In this connection, it is generally desirable to increase the magnitude of the damping force of a shock absorber when compressed in order to prevent the shock absorber from being fully compressed as when the vehicle travels over irregular ground surfaces but, for travelling over paved roads, the damping force during compression of a shock absorber should rather be limited as any excessively large damping force during compression impairs the riding comfort. Therefore, it is desired that the damping force of a shock absorber when compressed be freely variable in accordance with the ground or road conditions, that is, according to whether the vehicle travels over irregular surfaces or paved roads.
Also, unless the compression damping force and the expansion damping force of a shock absorber are properly adjusted relative to each other, an undesirable phenomenon is likely to occur in which the body of the vehicle gradually goes down or floats up, particularly when the vehicle travels over a rough road at relatively high speeds. Further, the effect upon the rider's feeling of the adjustment of the compression damping force is larger than that of the adjustment of the expansion damping force made at the same rate. Accordingly, it is desirable for a shock absorber to be arranged and constructed so that both the expansion and compression damping forces of the shock absorber are adjustable and that the range of adjustment of the compression damping characteristic and that of the expansion damping characteristic are made different from each other as required.
In addition, in view of the fact that such adjustment of the damping characteristics is to be effected frequently in accordance with the load, road and other driving conditions, the shock absorber should be of such a design as to require only a minimum of time or labor for adjustment of the damping characteristics.
Moreover, since in the suspension system any increase in unsprung weight should be avoided as far as possible, it is desirable to minimize any weight increase of a shock absorber as may be required to make variable its damping characteristics.